Investigations conducted during the initial support period of this grant suggest the existence of many subpopulations of thymus derived lymphoid cells. The objective of the proposed research is to identify these subsets of lymphoid cells on the basis of differential in vitro reactivity to antigens and phytomitogens; isolate and characterize them on the basis of physical and functional criteria; and to define the role of each subpopulation in the immune response of the intact animal.